In a receiver which receives data items it may be necessary to ensure that an individual data item is only received once. As such the receiver may be provided with the capability to detect duplicate (also known as “replayed”) data items, wherein the receiver makes reference to an identifier, such as a sequence number, associated with each data item, in order to determine if a newly received data item has in fact already been received by the receiver. In order to provide this functionality the receiver therefore needs to maintain a window of past sequence numbers against which a newly received sequence number can be checked.